Describe the normal electrocardiogram. Explain the leads, waves and intervals with their normal values, and give the uses of the ECG.

Definition

• Recorded at a paper speed of 25 mm/s and a sensitivity of 1 mV = 10 mm
• Therefore 1 small square = 0.04 s and 1 large square = 0.2 s

Leads

• Lead I = left arm − right arm; Lead II = left leg − right arm; Lead III = left leg − left arm
• Einthoven's law — Lead II = Lead I + Lead III
• AVR is normally negative in all its deflections

Regional correlation

Waves and Intervals

• Atrial repolarisation is not seen — it is buried within the QRS complex
• T wave is upright although it is repolarisation, because repolarisation proceeds from epicardium to endocardium, the reverse of depolarisation

Determination of Heart Rate and Axis

• Rate = 300 ÷ number of large squares between two R waves
• Or 1500 ÷ number of small squares
• Normal QRS axis = −30° to +90°
• Left axis deviation — LVH, left anterior hemiblock; right axis deviation — RVH, pulmonary embolism

Einthoven’s Triangle and the Cardiac Vector

• The three bipolar limb leads form an equilateral triangle with the heart at its centre
• The heart is regarded as a dipole in a volume conductor
• Cardiac vector — the resultant of all the instantaneous electrical forces in the heart, having magnitude and direction
• A wave is upright when depolarisation moves toward the positive electrode, and inverted when it moves away
• Mean electrical axis is calculated from leads I and aVF, or from the isoelectric lead

Uses of ECG

• Diagnosis of arrhythmias and heart block
• Diagnosis and localisation of myocardial infarction
• Detection of chamber hypertrophy
• Detection of electrolyte disturbances
• Assessing drug effects and toxicity (digitalis, antiarrhythmics)
• Determination of heart rate and electrical axis

Applied Aspects

• The ECG records electrical, not mechanical activity — a normal ECG can occur in a heart that is not pumping (pulseless electrical activity)